Galway Granite Research Articles

New light on the Oughterard Granite: Connemara's S-Type granite, Ireland

Abstract: Most of the Connemara granites, including the Galway Granite batholith, have a clearly defined circular or ellipsoidal outcrop shape and are I-Type granites often with K-feldspar phenocrysts, whereas the Oughterard Granite (OG) forms numerous small intrusions scattered east-west over 40km with two larger linked bodies near Oughterard and is S-Type and aphyric. The only comprehensive mapping, mineralogical and chemical study of the OG was published over 50 years ago before the important 1974 recognition of the distinction of I- and S-Type granites was first made. New mapping of the southern Oughterard area with small OG intrusions is presented and many scattered published chemical and Rb, Sr, S and Pb isotopic studies integrated to confirm overall their individual conclusions that the peraluminous OG is of S-Type, being formed from melted Dalradian rocks as are the fluid-carried associated uneconomic mineral deposits. The OG is confined to the high sillimanite zone, mostly south of, and mainly later than, the 465-464Ma D4 Connemara Antiform, where a wide (>15km N-S) >70km E-W band of 469Ma gabbros and 467Ma quartz diorite gneisses intruded into already hot Dalradian rocks undergoing regional metamorphism. This generated the OG magmas by partial melting of pelites and semi-pelites. The upward intrusion of the OG magma was significantly delayed, as has been shown by precise dating in other granites elsewhere, to ∼464-461Ma during the late stages and mostly after D4 but before the country rocks had completely cooled. The OG intrusions as a whole probably form the second largest area of S-Type granite in Ireland, after the Leinster Granite.

New Light on the Oughterard Granite: Connemara’s S-Type Granite, Ireland

Most of the Connemara granites, including the Galway Granite batholith, have a clearly defined circular or ellipsoidal outcrop shape and are I-Type granites often with K-feldspar phenocrysts, whereas the Oughterard Granite (OG) forms numerous small intrusions scattered east-west over 40km with two larger linked bodies near Oughterard and is S-Type and aphyric. The only comprehensive mapping, mineralogical and chemical study of the OG was published over 50 years ago before the important 1974 recognition of the distinction of I- and S-Type granites was first made. New mapping of the southern Oughterard area with small OG intrusions is presented and many scattered published chemical and Rb, Sr, S and Pb isotopic studies integrated to confirm overall their individual conclusions that the peraluminous OG is of S-Type, being formed from melted Dalradian rocks as are the fluid-carried associated uneconomic mineral deposits. The OG is confined to the high sillimanite zone, mostly south of, and mainly later than, the 465-464Ma D4 Connemara Antiform, where a wide (>15km N-S) >70km E-W band of 469Ma gabbros and 467Ma quartz diorite gneisses intruded into already hot Dalradian rocks undergoing regional metamorphism. This generated the OG magmas by partial melting of pelites and semi-pelites. The upward intrusion of the OG magma was significantly delayed, as has been shown by precise dating in other granites elsewhere, to ~464-461Ma during the late stages and mostly after D4 but before the country rocks had completely cooled. The OG intrusions as a whole probably form the second largest area of S-Type granite in Ireland, after the Leinster Granite.

A review of molybdenite, and fluorite mineralisation in Caledonian granite basement, western Ireland, incorporating new field and fluid inclusion studies, and Re-Os and U-Pb geochronology

The recent discovery of late-magmatic quartz vein hosted molybdenite, and exceptional gem quality vein fluorite, in the Caledonian Galway Granite Complex (GGC), has prompted a review of these contrasting styles of mineralisation in the late-Caledonian granite basement, Connemara, western Ireland. Existing published U-Pb and Re-Os chronometry and fluid inclusion microthermometry are combined with new: a) geological field observations, b) U-Pb zircon and Re-Os molybdenite geochronometry and c) fluid inclusion microthermometry to generate a new pressure-temperature-time model (P-T-t) of mineralisation for the GGC. Re-Os chronometry molybdenite indicates that granite related molybdenite mineralisation extended from 423 to 380Ma overlapping with the GGC emplacement history determined by U-Pb zircon chronometry. The P-T-t model reflects initial granite emplacement and Mo-mineralisation at ∼423Ma followed by lower P and T granite emplacement and related quartz vein hosted Mo-mineralisation at ∼410Ma (Carna pluton), ∼400Ma (Kilkieran pluton) and at ∼380Ma (Costelloe Murvey granite). The gem quality fluorite veins in the GGC represent late-Triassic hydrothermal mineralisation. These veins form part of a regional N Atlantic-European Triassic-Jurassic hydrothermal mineralisation province. Vein emplacement was triggered by the rifting of the N Atlantic facilitating crustal thinning and subsidence of continental crust and initiating hydrothermal activity at the margins of Mesozoic basins.

New U-Pb Zircon Ages for the Inish Granite Pluton, Galway Granite Complex, Connemara, Western Ireland

New U-Pb zircon ages are presented from two samples of the Inish Pluton, Galway Granite Complex (GGC), Connemara, western Ireland. One sample is from the Main Inish Granite and the other is from the pluton's marginal granite facies. The results of the U-Pb zircon geochronology show that the Main Inish Granite was emplaced at 422.72±0.19Ma and the marginal facies at 423.77±0.26Ma. The significance of the new ages is discussed in relation to the geology of the Inish Pluton and the temporal assembly of the GGC.

New U-Pb Zircon Ages for the Inish Granite Pluton, Galway Granite Complex, Connemara, Western Ireland

New U-Pb zircon ages are presented from two samples of the Inish Pluton, Galway Granite Complex (GGC), Connemara, western Ireland. One sample is from the Main Inish Granite and the other is from the pluton's marginal granite facies. The results of the U-Pb zircon geochronology show that the Main Inish Granite was emplaced at 422.72±0.19Ma and the marginal facies at 423.77±0.26Ma. The significance of the new ages is discussed in relation to the geology of the Inish Pluton and the temporal assembly of the GGC.

The Connemara Eastern Boundary Fault: A correction

THE CONNEMARA EASTERN BOUNDARY FAULT: A CORRECTION ALAN LEES and MARTIN FEELY (Received 16 August 2017. Accepted 4 September 2017.) Introduction In our paper published recently in this journal, we discussed the configuration of faults in the area south-west of Oughterard, Co. Galway (Lees and Feely 2016, pp 4–5). The information used was derived from an unpublished map by C.V. MacDermot dating from 1970 (copy in Philcox and Lees 2006) and from a publication by Bernard Leake (2006). The essentials of these two sets of information were presented in our Fig. 4, whose maps are reproduced here, with annotations, as Fig. 1. As regards the contact between the basement and the Carboniferous rocks, MacDermot recognised the existence of a NW–SE fault, the Oughterard Fault, the rest of the contact being unconformable (Fig. 1A). In contrast, Leake’s interpretation, given on a smallscale (1:140,000) synoptic geological map (Leake 2006, fig. 3) showed the contact as being defined by two faults (labelled faults 1 and 2 on Fig. 1B), one of which (fault 2) emerges from the basement and cuts across the other (fault 1) that could conceivably be regarded as a repositioned and extended Oughterard Fault. Correction Soon after the publication of our paper we were informed by Bernard Leake that it contained an error arising from figure 3 of his 2006 publication. This concerns the fault extending along the north-western side of the Cloosh embayment and turning to pass west of Oughterard (that is, fault 1 in Fig.1B), which he now acknowledges was ‘a cartographic error’ that he did not detect. The detailed coloured 1:25,000 map, figure 2 in his 2006 paper, does not show this fault. It simply marks the eastern limit of the Irish Journal of Earth Sciences 35 (2017), 55–56 doi: https://doi.org/10.3318/IJES.2017.35.55© 2017 Royal Irish Academy 55 Fig. 1.—Comparison of the maps of the basement/Carboniferous boundary in the Oughterard area. Reproduction of the maps of Fig. 4 of Lees and Feely (2016), with new annotations in blue. (A). MacDermot’s 1970 interpretation; (B).The relevant part of Leake’s synoptic map (2006, fig. 3). Fault 1 in map B is now acknowledged by Leake to be an error. 56 Irish Journal of Earth Sciences (2017) basement rocks, which, on the north-western side of the Cloosh Embayment, lies close to the line of the basement/Carboniferous unconformity tentatively mapped by MacDermot (Fig. 1A). In fact, Leake (pers. comm.) now agrees with MacDermot that: 1. the course of the Oughterard Fault passes south-eastwards from the waterfall exposure into the Carboniferous, and 2. the north-western flank of the Cloosh Embayment follows the unconformity at the base of the Carboniferous rocks. He believes, however, that his own mapping of the fault emerging from the basement to pass along the southern flank of the Embayment is correct. We have no reason to doubt these points. This revision is shown in the inset to Fig. 2 below. Fig. 11 of our paper needs the same modification. Fig. 2.—A simplified version of Fig. 10 of Lees and Feely (2016), showing the revised version of the faults in the Oughterard area, as detailed on the inset map, in the context of the Boundary Fault zone from Oughterard to Galway. The inset map is shown at the same scale as the maps of Fig. 1. References Leake, B.E. 2006 Mechanism of emplacement and crystallisation history of the northern margin and centre of the Galway Granite, western Ireland. Transactions Royal Society of Edinburgh: Earth Sciences 97, 1–23. Lees, A. and Feely, M. 2016 The Connemara Eastern Boundary Fault: a review and assessment using new evidence. Irish Journal of Earth Sciences 34, 1–25. Philcox, M.E. and Lees, A. 2006 The Lower Carboniferous Rocks of G.S.I. map sheets 11 & 14: a dossier of source materials for the Geological Survey of Ireland. (.doc version, c. 185 pp). Lodged with the Geological Survey of Ireland. ALAN LEES Bleanoran, Knockferry, Roscahill, Co. Galway H91 AD6F E-mail bleanoran16@gmail.com MARTIN FEELY Earth and Ocean Sciences School of Natural Sciences National University of...

Late Caledonian transpression and the structural controls on pluton construction; new insights from the Omey Pluton, western Ireland

ABSTRACTThe Galway Granite Complex is unique among the British and Irish Caledonian granitoid terranes, as it records punctuated phases of magmatism from ∼425–380 Ma throughout the latest phase of the Caledonian Orogeny. Remapping of the Omey Pluton, the oldest member of this suite, has constrained the spatial distribution and contact relationships of the pluton's three main facies relative to the nature of the host rock structure. The external contacts of the pluton are mostly concordant to the limbs and hinge of the Connemara Antiform. New AMS data show that a subtle concentric outward dipping foliation is present, and this is interpreted to reflect pluton inflation during continued magma ingress. Combined field, petrographic and AMS data show that two sets of shear zones (NNW–SSE and ENE–WSW) cross-cut the concentric foliation, and that these structures were active during the construction of the pluton. We show that regional sinistral transpression at ∼420 Ma would have caused dilation along the intersection of these two fault sets, and suggest that this facilitated centralised magma ascent. Lateral emplacement was controlled by the symmetry of the Connemara Antiform to ultimately produce a discordant phacolith. We propose that regional sinistral transpression at ∼420 Ma influenced the siting of smaller intrusions over NNW–SSE faults, and that the later onset of regional transtension caused larger volumes of magma to intrude along the E–W Skird Rocks Fault at ∼400 Ma.

Glacier Dynamics around Slieve Bloom, Central Ireland

Analyses of both till and glacioaqueous sedimentary fabrics and their lithological characteristics have been carried out on sediments sampled at eight sites located around the northern margin and piedmont of Slieve Bloom. Results indicate that Slieve Bloom was overtopped by ice (lowing locally from between the south-south-west and west. The pattern of ice (low indicated by the sedimentary fabrics, the presence in the glacigenic sediments of a distinctive variety of Galway granite and the spatial variability of limestone in these sediments indicate that this local flow direction resulted from the deflection to the north-north-east of ice (lowing from the west. As the most widely accepted ice flow model for this region during the last glaciation requires ice (low to have been from between north-north-west and north-north-east, the results presented in this paper have important implications for the modelling of regional ice (low in the Irish midlands during the last glaciation.

Major Thrusting in the Granite and the Role of Late Intrusions in Exposing the Deeper Parts of the Central Block of the Galway Granite Batholith

Major Thrusting in the Granite and the Role of Late Intrusions in Exposing the Deeper Parts of the Central Block of the Galway Granite Batholith

MAJOR THRUSTING IN THE GRANITE AND THE ROLE OF LATE INTRUSIONS IN EXPOSING THE DEEPER PARTS OF THE CENTRAL BLOCK OF THE GALWAY GRANITE BATHOLITH

New geological mapping in the centre of the Galway Granite at Camus, Co. Galway, reveals a major north—south thrust zone, the Furnace Thrust, typically dipping east at 35°, which thrust up westward deeper-crystallised granite, including two minor late intrusions, on top of a higher-level granite footwall. Al-in-hornblende geobarometry of the footwall and the hanging wall confirms the thrusting. At 2km east of the thrust, both the footwall and the hanging wall of the thrust sheet were later substantially further uplifted by the steeper Shannawona Fault. At 6km east of this fault, the granite contains post-400—395Ma, pre-370—378Ma thrusts that moved south-eastward, so a major block of the deep ∼400—395Ma Megacrystic Granite has been squeezed upward. This granite still farther east was later uplifted more by the steep Shannapheasteen Fault, which is connected with the late (?380Ma) central intrusion of the Shannapheasteen Granite, which pushed its roof upward. The uplift of the Central Block with its deeply crystallised Megacrystic Granite was therefore the result of thrusts and faults connected in a complex way with the coeval intrusive pressures of the late emplacement of the Shannapheasteen Granite and six other late granites. All seven late granites are confined to the Central Block and, having low densities, exerted protracted buoyancy uplift forces. This modifies the previous partly correct, but mechanically difficult, explanation for the origin of the Central Block of the Galway Batholith.

Stoping and the mechanisms of emplacement of the granites in the Western Ring Complex of the Galway granite batholith, western Ireland

ABSTRACTThe western end of the Galway granite batholith demonstrates the importance of stoping as a granite emplacement process, which is currently controversial, and also of space generation by uplift of the centre of a ring complex. The granite rings are shown (with a coloured 1:25 000 geological map) to be consanguineous, near coeval, and older than the 407–410 Ma late molybdenite mineralisation. A newly-recognised Mace–Ards granite, around and injected by the Aplitic Murvey-type granite of the ring core (both lacking hornblende and titanite), has biotite–muscovite–cordierite orbs and sulphide–granite orbs, showing separation of immiscible hydrous and sulphide fluids from the late magma which, with vugs, indicates a low pressure, near-roof site. The outer ring of the Errisbeg Townland granite (ETG, the main batholith granite with K-feldspar phenocrysts), was emplaced by progressive outward stoping of the country rock metagabbro, as shown by mapping, and by chemical fractionation of feldspars, biotites and bulk rocks, to the marginal, dry, fine-grained aphyric, in part garnetiferous, highly fractionated, siliceous Murvey granite. Stoping ceased when, after previously invading dense metagabbro, the outer ring complex reached the low-density Roundstone granite, which is shown for the first time to be older than the Galway batholith. This arresting of the batholith intrusion shows that stoping was such a significant process that emplacement ceased when stoping became impossible. The inside edge of the ETG grades into the slightly later, intrusive, aphyric Carna granite, which shows inward fractionation to the wet magma of the Mace–Ards granite. The ring complex core was injected by highly fractionated, dry, Aplitic Murvey-type granite, intensely hydrothermally altered by late magmatic water. The radially outward dipping, inclined igneous layering in the ETG shows that the original ETG centre was pushed upwards by the intruded Carna granite and eroded away. The Galway granite and its nearby magmatism matches the low Ba and Sr, high Th and Rb, Scottish Cairngorm Suite and similarly has few appinitic rocks associated with it. Magmatism extended over >45 Myr from ∼425 Ma to 380 Ma. It originated by slab breakoff and consequent rise of the asthenosphere, causing deep crustal melting.

Long-lived granite-related molybdenite mineralization at Connemara, western Irish Caledonides

AbstractNew Re–Os age determinations from the Galway Granite (samples: KMG = 402.2 ± 1.1 Ma, LLG = 399.5 ± 1.7 Ma and GBM = 383.3 ± 1.1 Ma) show that in south Connemara, late Caledonian granite-related molybdenite mineralization extended from c. 423 Ma to c. 380 Ma. These events overlap and are in excellent agreement with the published granite emplacement history determined by U–Pb zircon geochronology. The spatial distribution of the late-Caledonian Connemara granites indicates that initial emplacement and molybdenite mineralization occurred at c. 420 Ma (that is, the Omey Granite and probably the Inish, Leterfrack and Roundstone granites) to the N and NW of the Skird Rocks Fault, an extension of the orogen-parallel Southern Uplands Fault in western Ireland. A generally southern and eastward progression of granite emplacement (and molybdenite mineralization) sited along the Skird Rocks Fault then followed, at c. 410 Ma (Roundstone Murvey and Carna granites), at c. 400 Ma (Errisbeg Townland Granite, Megacrystic Granite, Mingling Mixing Zone Granodiorite, Lough Lurgan Granite and Kilkieran Murvey Granite) and at c. 380 Ma (Costelloe Murvey Granite, Shannapheasteen and Knock granites). The duration of granite magmatism and mineralization in Connemara is similar to other sectors of the Appalachian–Caledonian orogeny and several tectonic processes (e.g. slab-breakoff, asthenospheric flow, transtension and decompression) may account for the duration and variety of granite magmatism of the western Irish Caledonides.

Till sedimentology and stratigraphy on the Dingle Peninsula, SW Ireland: implications for Late Quaternary regional ice flow patterns

Investigation of Late Quaternary glacigenic sediments exposed in coastal sections on the Dingle Peninsula, southwest Ireland shows that during the last glaciation, an advance of local ice occurred across the western and northern coasts of the peninsula and deposited a series of subglacial tills. These tills were formed largely by the subglacial reworking of underlying parent material. Sedimentological contrasts between them reflect variations in the nature of the parent material from which the tills were derived, as well as variations in the magnitude of strain during till formation. The local ice reworked pre-existing glacial deposits associated with an earlier advance of regional ice from the north. Evidence for this earlier advance of regional ice is provided by erratics of Galway Granite contained within the tills at some sites. No in-situ deposits associated with the granite-carrying ice were observed on the peninsula. Clast macrofabric and provenance data indicate that all of the tills recorded are the product of the radial flow of local ice coastwards from the peninsula.

Mechanism of emplacement and crystallisation history of the northern margin and centre of the Galway Granite, western Ireland

ABSTRACTThe main phase (∼400 Ma) emplacement of the central and northern part of the reversely zoned Galway Granite was incremental by progressive northward marginal dyke injection and stoping of the 470–467 Ma Connemara metagabbro-gneiss country rock. The space was provided by the synchronous ESE-opening, along the strike of the country rocks, of extensional fractures generated successively northward by a releasing bend in the sinistrally moving Skird Rocks Fault or an equivalent Galway Bay Fault. This fault is a prolongation of the Antrim–Galway (a splay off the Highland Boundary Fault) and Southern Upland Faults. The ESE-strike of the spalled-off rocks controlled the resultant ESE-elongated shape of the batholith. The magma pulses (∼5–30 m in thickness) were progressively more fractionated towards the northern margin so that the coarse Porphyritic (or Megacrystic) Granite (GP; technically granodiorite) in the centre was followed outwards by finer grained, drier and more siliceous granite, until the movements opening the fractures ceased and the magma became too viscous to intrude. ‘Out-of-sequence’ pulses of more basic diorite-granodiorite (including the Mingling–Mixing Zone) and late main phase, more acid, coarse but Aphyric Granite, into the centre of the batholith, complicated the outward fractionation scheme. The outward expansion, caused by the intrusions into the centre, caused a foliation and flattening of cognate xenoliths within the partly crystallised northern marginal granite and in the Mingling–Mixing Zone to the south.Late phase (∼380 Ma) central intrusions of the newly-discovered aphyric Shannapheasteen Finegrained Granite (technically granodiorite), the Knock, the Lurgan and the Costello Murvey Granites, all more siliceous and less dense than the GP, were emplaced by pushing up the already solid and jointed GP along marginal faults. This concentration of lighter granites plus compression shown in thrusting, caused overall fault uplift of the Central Block of the Galway batholith so that the originally deepest part of the GP is exposed where there is the most late phase granite. Chemical analyses show the main and late phase magmas, including late dykes, were very similar, with repetition of the same fractionation except that the late phase magmas were drier and more quickly cooled, giving finer grained rocks.

Mechanism of emplacement and crystallisation history of the northern margin and centre of the Galway Granite, western Ireland

Mechanism of emplacement and crystallisation history of the northern margin and centre of the Galway Granite, western Ireland

A microstructural and fabric study of the Galway Granite, Connemara, western Ireland

A detailed field and microstructural investigation of mineral fabrics in the late-Caledonian Galway Granite Batholith (∼400 Ma) provides insights into the relationship between emplacement-related deformation and crystallization state. These relationships are used to infer the regional instantaneous strain pattern at the time of intrusion. A Marginal Deformation Zone occurs in the granite along part of its northern sector, where planar fabrics are contact-parallel and dip steeply to the north. Within the Marginal Deformation Zone, the granite has similar patterns of pre- and post-RCMP (Rheologically Critical Melt Percentage) fabrics on either side of the NNE-trending Shannawona Fault Zone, which separates the Western and Central blocks of the batholith. Oblate pre-RCMP fabrics, which intensify towards the contact, are overprinted in a down-temperature continuum of deformation by co-axial post-RCMP fabrics that also become more intense towards the contact. At the southern edge of the Marginal Deformation Zone, deformation ceased before the granite reached its RCMP whereas, close to the contact, deformation ceased at ∼500°C. Within the Central Block, oblate fabrics also developed parallel with internal granite facies boundaries. Throughout the batholith, the fabrics formed by co-axial deformation as a result of lateral expansion operating in successive magma batches at the emplacement level. These intrusion-related fabrics are consistent with other evidence that indicates the Galway Granite was emplaced into a transtensional setting at the end of the Caledonian Orogeny.

Reply to “Accurate and precise Re-Os molybdenite dates from the Galway Granite, Ireland” by D. Selby et al.: Critical comment on “Disturbance of the Re-Os chronometer of molybdenites from the late-Caledonian Galway Granite, Ireland, by hydrothermal fluid circulation”

of our previous papers (Suzuki et al., 2000, 2001) was focussed on improving our understanding of the behaviour of Re and Os. Therefore, we sampled a part of molybdenite crystal(s) and analyzed each part. This allowed us to examine the micro-scale movement of Re and/ or 187Os and was similar to the later LA-ICP-MS spot dating studies (Kosler et al., 2000; Stein et al., 2003) in concept. Irreproducible Re-Os dates would be expected using such micro-sampling techniques in Cases (b) and (c). Consequently, the sampling methodology promoted by Stein and co-workers and Selby and co-workers would have been inappropriate as bulk sampling averages out any variations present on a scale smaller than the bulk sample and it was precisely these variations we wished to study. Thus, I dispute that our sample preparation technique was “inadequate”, for our purpose. I would argue that Re-Os system disturbance in molybdenite has been shown to have occurred in certain environments. McCandless et al. (1993) found that certain molybdenites experienced Re loss during hydrothermal alteration in hypogene and supergene environments and that Re was enriched in K-Al-silicate intergrowths. They also proposed that Re may be enriched in powellite, an alteration product of molybdenite, in the weathering environment. On the other hand, we (Suzuki et al., 2000) experimentally showed that the outer parts of molybdenite crystals exposed to 0.1 mol/L NaCl and NaHCO3 solutions produced younger Re-Os dates than that of the original unaltered molybdenite, while the inner parts of the molybdenites yielded dates in agreement with the original within analytical uncertainties. These results indicated that the molybdenite lost Re and/or 187Os during experimental alteration. Also, Suzuki et al. (1994) had previously reported that approximately 5% of Re was found to dissolve into the altering NaCl solution, which also indiI welcome the opportunity to reply to the critical comments of Selby et al., which may invite much concern about the Re-Os isotope system. I hope that we have a fruitful discussion in Geochemical Journal. Prior to opening my reply, I exhibit 4 possible cases of Re and Os behaviour in molybdenite and the resulting apparent Re-Os ages (Fig. 1). Case (a): No movement of either Re or radiogenic 187Os after formation of molybdenite. Case (b): Movement of Re and/or 187Os “within” a molybdenite crystal (decoupling). Case (c): Movement of Re and/or 187Os through the crystal surface, but not through the whole rock system. Case (d): Movement of Re and/or 187Os through the whole rock system. Obviously, the Re-Os system is undisturbed in Case (a) and thus a robust Re-Os age can be obtained. In Case (b), we can obtain a reliable age, although the Re-Os system is disturbed within individual crystals. Each crystal should yield a concordant Re-Os age, because the Re-Os system has remained closed within the crystal. Case (c) presents the more complicated situation where Re and/or 187Os have moved beyond the crystal surface. Since the whole rock system has remained closed, a reliable Re-Os age may still be obtained if complete collection of the molybdenite sample is possible. In Case (d), Re and/or Os have moved in an open system and a robust Re-Os age is not possible.

Accurate and precise Re-Os molybdenite dates from the Galway Granite, Ireland. Critical comment on “Disturbance of the Re-Os chronometer of molybdenites from the late-Caledonian Galway Granite, Ireland, by hydrothermal fluid circulation” by Suzuki et al., Geochemical Journal, 35, 29-35, 2001

Accurate and precise Re-Os molybdenite dates from the Galway Granite, Ireland. Critical comment on “Disturbance of the Re-Os chronometer of molybdenites from the late-Caledonian Galway Granite, Ireland, by hydrothermal fluid circulation” by Suzuki et al., Geochemical Journal, 35, 29-35, 2001

Late Magmatism of the Galway Granite Batholith: II. Composite Dolerite–Rhyolite Dikes

Composite dolerite-rhyolite dikes traverse the Galway Granite batholith and its adjacent envelope. The dikes pertain to the Teach D6ite suite and were previously considered to be of Carboniferous age. New and extended examination of field relationships supports recent radiometric dating for an intrusive period that overlapped with the final consolidation of the Galway batholith. Regional crustal extension produced a complex pattern of fissuring, controlled by various preexisting structures, which permitted ascent of mantle-derived melts into and around the Galway batholith. Ponding of mafic magma at an intermediate level facilitated crustal partial melting and the generation of high-silica, high-alumina rhyolitic melts. The two contrasting magmas then rose into common or proximate dike fissures, rhyolitic injection immediately following that of dolerite. Magma storage in stratified chambers occasionally resulted in the development of a hybrid magma layer, but in all cases minor mingling and mixing beween dolerite and rhyolite magma continued up into the dikes. Rhyolite geochemistry precludes a genetic relationship with the Galway granitoids, despite a few instances where granitic material was entrained into rhyolitic magma. Introduction and setting The 400Ma Galway Granite batholith was emplaced into 470Ma island-arc orthogneisses in the Connemara sector of the Caledonides. This emplacement was followed by the intrusion of two hypabyssal suites: earlier microphyric ('porphyry') dacite dikes (Kinahan 1869; Mohr 2003) and a later complex nexus of dolerite dikes, the Teach D6ite (TD) suite (Mitchell and Mohr 1987; Fig. 1). The numerous and widespread dacite dikes have consistently been considered the youngest igneous rocks pertaining to the Galway batholith (Wager 1932; Wright 1961; Harvey 1967; Coats and Wilson 1971; Senior 1973; Leake 1974). However, new work summarised here suggests that the subsequent TD dikes were a final manifestation of the magmatic episode responsible for the batholith. The regional pattern of TD dikes comprises three major linear trends (Fig. 1). In the central part of the Galway batholith and its northern envelope, the NNE-trending Seanabhain system of dikes is intimately associated with the Shanawon Fault that separates the central and western blocks of the batholith (Feely and Madden 1988; Mohr 1993; Callaghan 1999). The na hUillinni dike system, 3.5km west of the Seanabhan system and parallel to it, projects much farther NNE into the orthogneiss envelope (Fig. 1). Secondly, a grid of ENE-trending dikes Irish Journal of Earth Sciences 22 (2004), 15-32. © Royal Irish Academy 15 This content downloaded from 207.46.13.174 on Sun, 10 Jul 2016 05:04:50 UTC All use subject to http://about.jstor.org/terms 16 Irish Journal of Earth Sciences (2004)

U-Pb zircon geochronology of the Galway Granite, Connemara, Ireland: implications for the timing of late Caledonian tectonic and magmatic events and for correlations with Acadian plutonism in New England

The Galway Granite occupies a key location in the Caledonides of western Ireland. The 80 km long, WNW-trending axis of the batholith lies astride and stitches the EW-trending Skerd Rocks Fault, a splay of the orogen-parallel Southern Uplands Fault. The Skerd Rocks Fault separates high-grade metamorphic rocks of the Connemara Massif from Lower Ordovician greenschist-facies metavolcanic and metasedimentary rocks. We present new isotopic U-Pb zircon age determinations, using thermal ionisation mass spectrometry (TIMS), for five granite samples from the central block of the Galway Granite. Early coeval diorite and granite magmas are concomitant with the development of pure flattening fabrics and are succeeded by emplacement of a suite of generally unfoliated granites intruded in a brittle fracture regime. The results of the U-Pb zircon geochronology (G1 = 394.4 ± 2.2 Ma; G2~402 Ma; G3 = 397.7 ± 1.1 Ma; G4 = 399.5 ± 0.8 Ma) indicate that the emplacement of the early suite occurred over at least 8 Ma and belongs to the Late Emsian stage. Samples of commingled diorite (G3) and granite (G4) yield ages that are indistinguishable within error. Sample G5 is from an alkali feldspar leucogranite that cuts the early suite. Zircon from this granite yielded a U-Pb age of 380 ± 6 Ma and a single concordant monazite yielded 383.6 Ma, indicating emplacement at the Givetian-Frasnian boundary and a gap of ~20 Ma between intrusion of the two granite suites. These data provide constraints on the timing of final movement on the orogen-parallel strike-slip Southern Uplands-Skerd Rocks Fault System, and have implications for correlations between Acadian plutonism in New England and in western Ireland. RÉSUMÉ Le granite de Galway occupe un emplacement clé dans les Calédonides de l'ouest de l'Irlande. Le batholite de 80 kilomètres de longueur suivant un axe ouest-nord-ouest chevauche et suture la faille orientée d'est en ouest des rochers Skerd, un évasement de la faille parallèle à l'orogène des Hautes terres du Sud. La faille des rochers Skerd sépare des roches métamorphiques à forte teneur du massif de Connemara de roches métasédimentaires et métavolcaniques d'un faciès de schistes verts de l'Ordovicien inférieur. Nous présentons de nouvelles datations isotopiques au U-Pb obtenues à partir de zircon, au moyen de la spectrométrie de masse à ionisation thermique (SMIT), de cinq échantillons de granite provenant du bloc central du granite de Galway. Les premiers magmas granitiques et dioritiques contemporains sont apparus en même temps que se sont établis les fabriques d'aplanissement pures et ils ont été suivis de l'intrusion d'un cortège de granites généralement non feuilletés pénétrant dans un régime de rupture fragile. Les résultats de l'exercice géochronologique au U-Pb à partir de zircon (G1 = 394,4 ± 2,2 Ma; G2 ~ 402 Ma; G3 = 397,7 ± 1,1 Ma; G4 = 399,5 ± 0,8 Ma) révèlent que l'intrusion du premier cortège s'est échelonnée au cours d'au moins 8 Ma et qu'elle remonte au stade de l'Emsien tardif. Des échantillons de diorite (G3) et de granite (G4) mélangés ont présenté des âges qu'il est impossible de distinguer sans erreur. L'échantillon G5 provient d'un leucogranite de feldspath alcalin qui entrecoupe le cortège initial. La datation au U-Pb à partir de zircon de ce granite a révélé qu'il était âgé de 380 ± 6 Ma et un échantillon de monazite concordant unique a été situé à 383,6 Ma, un âge témoignant d'une mise en place à la limite du Givétien et du Frasnien et d'un écart d'environ 20 Ma entre l'intrusion des deux cortèges granitiques. Ces données imposent des contraintes à la localisation chronologique du déplacement sur les groupements de décrochements parallèles à l'orogène des Hautes terres du Sud-rochers Skerd, et elles ont des répercussions sur les corrélations entre le plutonisme acadien en Nouvelle-Angleterre et dans l'ouest du l'Irlande. [Traduit par la rédaction]